The use of contact lenses for the correction of visual acuity, for cosmetic purposes, or both is well known. Soft contact lenses are known, which lenses generally are manufactured from materials including, without limitation, hydrogels and silicone hydrogels.
In normal use on the lens wearer's eye, a portion of the soft contact lens overlies the wearer's cornea. Corneal cell damage that is observable by corneal staining, or upon instillation of a diagnostic dye onto the corneal surface, may occur in soft contact lens wearers. Of particular interest is staining resulting from mechanical chaffing of the lens on the cornea. Such staining is considered a precursor to corneal tissue splitting. Such damage has been observed in wearer's of lathe cut lenses, lenses made from materials having a Young's modulus of 40 psi or greater, lenses with a single back surface curve, and lenses in which the center thickness, the peripheral thickness profile, or cross-sectional thickness, or both is greater than about 0.1 mm.
Additionally, corneal cell damage has been observed in wearer's of lenses having certain geometries on their front or back surfaces. Particularly problematic geometries are those in which junctions between zones on the front or back surface of the lens creates surface discontinuities in the mid-peripheral area of the lens. For purposes of the invention, by mid-peripheral area is meant the non-optical area, outside of the optical zone, between the lens' geometric center and the lens edge, which area overlies the cornea when the lens is on-eye. Typically, this area will have a radius of between about 3.5 and 6.5 mm from the lens' geometric center. Finally, corneal cell damage can result from lenses having areas over which the thickness profile is non-constant.
Corneal staining due to mechanical chaffing is most commonly found in the superior region of the cornea, or the area of the cornea above the pupil, and in the inferior region, or region below the pupil. Thus, the cell damage is most likely to appear in the areas of the cornea that underlie the upper or lower eyelid. The incidence of corneal staining varies from lens to lens based on the material and design of the lens, but can be as high as 50% of lens wearers. Therefore, a need exists for lens designs in which the incidence of corneal staining is reduced or eliminated.